System and Method for Treatment of Acidic Wastewater

ABSTRACT

A method for removing contaminants from an influent wastewater stream includes initially chemically adjusting a pH of the influent wastewater stream to less than about 3.5 or maintaining the pH of the influent stream at less than about 3.5. After initially adjusting or maintaining the pH of the influent wastewater stream, the wastewater is directed to a first reverse osmosis system and contaminants are removed from the wastewater. The wastewater is then directed to a second reverse osmosis where additional contaminants are removed from the wastewater. After the wastewater has been subjected to treatment in the first reverse osmosis system and prior to treatment in the second reverse osmosis system, the pH of the wastewater is adjusted upwardly.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. § 120 from thefollowing U.S. patent application Ser. No. 10/899,326 filed on Jul. 26,2004, which claims priority under 35 U.S.C. § 119(e) from the followingU.S. provisional patent Application Ser. No. 60/489,853 filed on Jul.24, 2003. Both applications are incorporated by reference herein. Thepresent application is a continuation of U.S. patent application Ser.No. 10/899,326.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to treatment of acidicindustrial wastewater and, more particularly, to minimizingprecipitation in reverse osmosis systems utilized to treat wastewater.

2. Discussion of the Related Art

Wastewater associated with phosphate manufacturing operations istypically acidic and typically has fluoride, ammonia, silica, sulfate,calcium, heavy metal and phosphate species. Various techniques have beenutilized to reduce the level of such contaminants before wastewater canbe discharged. For example, the double liming process, followed by airstripping, is a technique that is typically used. It utilizes limeaddition in two stages, to promote precipitation of fluoride species andphosphate species, followed by high pH, air stripping to remove ammonia.In another technique, wastewater has been treated by techniquesinvolving chemical precipitation followed by reverse osmosis. Like thedouble liming process, such techniques raise the pH of influentwastewater to promote precipitation and solids separation before thereverse osmosis step. The high chemical costs typically associated withraising the pH of the wastewater make such processes economicallyunattractive.

BRIEF SUMMARY OF THE INVENTION

In accordance with one or more embodiments, the present inventionprovides a wastewater treatment system comprising an influent sourcecomprising wastewater to be treated having a pH less than about 3.5, afirst reverse osmosis system fluidly connected to the influent source,an alkali source disposed to introduce alkali downstream of the firstreverse osmosis system, and a second reverse osmosis system fluidlyconnected downstream of the first reverse osmosis system and the alkalisource.

In accordance with one or more embodiments, the present inventionprovides a method of treating wastewater having a pH less than about3.5. The method comprises steps of removing at least a portion of anycontaminant from the wastewater in a first separation system, adjustingthe pH of an effluent from the first separation system to at least about6 or higher after removing at least a portion of any contaminant fromthe wastewater in the first separation system, and removing at least aportion of any contaminant from the wastewater in a second system afteradjusting the pH of the effluent from the first separation system to atleast about 6 or higher.

In accordance with one or more embodiments, the present inventionprovides a method of treating wastewater. The method comprises steps ofinhibiting conditions in the wastewater that promote the formation of atleast one of fluoride ions and silicate ions, removing any contaminantfrom the wastewater in a first separation system, promoting formation ofat least one of the fluoride ions and silicate ions, and removing anycontaminant from the wastewater to produce a treated effluent afterpromoting formation of at least one of the fluoride and silicate ions.

In accordance with one or more embodiments, the present inventionprovides a method of treating wastewater. The method comprises steps ofmaintaining an equilibrium condition for any precipitating contaminantin the wastewater, removing any one of phosphates, dissolved solids,ammonia, organic, and colloidal material from the wastewater, adjustingthe equilibrium condition of at least one precipitating contaminant inthe wastewater after removing any one of dissolved solids, ammonia,organic, and colloidal material from the wastewater, and removing anyresidual fluoride, ammonia, or dissolved solid material from thewastewater to produce a treated effluent after adjusting the equilibriumcondition of at least one precipitating contaminant in the wastewater.

The present invention provides a method of removing fluorides and silicafrom wastewater using a reverse osmosis system where the method reducesthe potential for scaling in the reverse osmosis system. In the case ofthis aspect of the invention, the method entails promoting conditions inthe wastewater that favor the formation of hydrofluorosilicic acid anddirecting the wastewater having the hydrofluorosilicic acid to thereverse osmosis system. As the wastewater passes through the reverseosmosis system, fluorides and silica in the form of thehydrofluorosilicic acid is removed from the wastewater. A second stagereverse osmosis system can be utilized to remove additional fluoridesand silica. In this case, conditions are maintained in the wastewatereffluent from the first reverse osmosis system that favors the formationof fluoride and silicate ions. Thus, additional fluorides and silica inthe form of fluoride and silicate ions are removed as the wastewaterpasses through the second reverse osmosis system.

Further, the present invention entails removing algae from wastewater.In one particular embodiment, the wastewater is acidic. To remove algaefrom the wastewater, chlorine or a byproduct of chlorine is added to thewastewater to kill the algae. Further, bentonite is added and the algae,after being subjected to treatment with the chlorine or chlorinebyproduct, is absorbed and or destabilized by the bentonite. Thereafterthe algae can be removed by conventional process means.

In one particular embodiment of the present invention, the algae and/orsuspended matter is removed through a ballasted flocculation separationsystem. In this process, the absorbed algae and bentonite form solids inthe wastewater. In the ballasted flocculation process, a flocculant andinsoluble granular material are added to the wastewater to form aflocculated mixture. The flocculated mixture form flocs, including theabsorbed algae and bentonite that settle from the wastewater.

Other advantages, novel features, and objects of the invention willbecome apparent from the following detailed description of the inventionwhen considered in conjunction with the accompanying drawings, some ofwhich are schematic and which are not intended to be drawn to scale. Inthe figures, each identical or nearly identical component that isillustrated in various figures is represented by a single numeral. Forpurposes of clarity, not every component is labeled in every figure, noris every component of each embodiment of the invention shown whereillustration is not necessary to allow those of ordinary skill in theart to understand the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting embodiments of the present invention will be described byway of example with reference to the accompanying drawings in which:

FIG. 1 is a process flow diagram in accordance with one or moreembodiments of the present invention showing a wastewater treatmentsystem;

FIG. 2 is a schematic diagram of a ballasted separation system inaccordance with one or more embodiments of the present invention;

FIG. 3 is a graph showing the equilibrium relative composition ofsulfate and bisulfate species as a function of pH in accordance with oneor more embodiments of the present invention;

FIG. 4 is a graph showing the equilibrium relative composition ofhydrofluoric acid and fluoride species as a function of pH in accordancewith one or more embodiments of the present invention;

FIG. 5 is a graph showing the equilibrium relative composition ofammonium and ammonia species as a function of pH; and

FIG. 6 is a graph showing the equilibrium relative composition ofphosphoric acid and phosphate species as a function of pH.

DETAILED DESCRIPTION OF THE INVENTION

Treatment of wastewater containing silica, calcium sulfate, calciumphosphate, calcium fluoride as well as any other species that canprecipitate under neutral, or near neutral, pH conditions presentscaling concerns. For example, reverse osmosis unit operations orsystems develop scale when such wastewater is passed therethrough. Otherpotential fouling problems include those associated with soluble organiccompounds as well as from organic materials. Consequently, such systemsface significant operating costs such as, but not limited to, membranecleaning and/or replacement and high chemical consumption. Accordingly,the present invention provides a system and a process for treatingwastewater that utilize chemical equilibrium properties in stages toproduce an effluent suitable for discharge in regulated waterways. Forexample, the system and methods in accordance with the present inventioncan produce effluent, treated wastewater, having low concentrations ofdissolved solids, fluoride, ammonia, phosphate, and sulfate species thatcan meet water discharge requirements. Thus, in accordance with one ormore embodiments, the present invention provides a wastewater treatmentsystem comprising an influent source comprising wastewater to be treatedhaving a pH less than about 3.5, a first reverse osmosis system fluidlyconnected to the influent source, an alkali source disposed to introducealkali downstream of the first reverse osmosis system, and a secondreverse osmosis system fluidly connected downstream of the first reverseosmosis system and the alkali source. The wastewater treatment systemcan further comprise a clarifier fluidly connected between the influentsource and the first reverse osmosis system. The wastewater treatmentsystem can further comprise a multimedia or other type of filter fluidlyconnected between the influent source and the first reverse osmosissystem. The wastewater treatment system can also further comprise anacid source disposed to add acid to the wastewater upstream of the firstreverse osmosis system. The wastewater treatment system can also furthercomprise a mixed-bed polisher fluidly connected downstream of the secondreverse 25 osmosis system. In accordance with further embodiments, thepresent invention provides a method of treating wastewater having a pHless than about 3.5. The method can comprise steps of removing at leasta portion of any undesirable species from the wastewater in a firstseparation system, adjusting the pH of an effluent from the firstseparation system to at least about 6 after removing at least a portionof any undesirable species from the wastewater in the first separationsystem, and removing at least a portion of any undesirable species fromthe wastewater in a second system after adjusting the pH of the effluentfrom the first separation system to at least about 6. The method canfurther comprise a step of clarifying the wastewater prior to performingthe step of removing at least a portion of any undesirable species inthe first separation unit operation. The method can further comprise astep of removing any organic matter from the wastewater prior toperforming the step of removing at least a portion of any undesirablespecies in the first separation system. The step of removing any organicmatter can comprise adding a disinfectant, a coagulant and aflocculating agent to the wastewater. The method can further comprise astep of removing any fine solids from the wastewater prior to performingthe step of removing at least a portion of any undesirable species inthe first separation system. The method can further comprise a step ofadjusting a pH of the wastewater to about 3 prior to performing the stepof removing at least a portion of undesirable species in the firstseparation system. The method can further comprise a step of reducingany one of ammonia and phosphate in treated wastewater from the secondseparation system to levels that comply with established EPArequirements.

In accordance with still further embodiments, the present inventionprovides a method of treating wastewater. The method can comprise stepsof inhibiting conditions in the wastewater that promote the formation ofat least one of fluoride ions and silicate ions, promoting conditions inthe wastewater that form or maintain a complexing species of silica andfluoride, removing at least one undesirable species from the wastewaterwhile promoting condition that form or maintain a complexing species ofsilica and fluoride, adjusting the wastewater conditions to inhibit theformation of the complexing species after removing at least oneundesirable species from the wastewater. The method can further comprisea step of removing at least a portion of any organic matter from thewastewater prior to removing any undesirable species from the wastewaterin a first separation system.

In accordance with other embodiments, the present invention provides amethod of treating wastewater. The method can comprise steps ofmaintaining an equilibrium condition for any precipitating species inthe wastewater, removing any one of dissolved solids, ammonia, organic,and colloidal material from the wastewater, adjusting the equilibriumcondition of at least one precipitating species in the wastewater afterremoving any one of dissolved solids, ammonia, organic, and colloidalmaterial from the wastewater, and removing any residual fluoride,ammonia, or dissolved solid material from the wastewater to produce atreated effluent after adjusting the equilibrium condition of at leastone precipitating species in the wastewater. The step of removing anyone of dissolved solids, ammonia, organic, and colloidal material fromthe wastewater can be performed while maintaining an equilibriumcondition for any precipitating species in the wastewater. In accordancewith yet other embodiments, the present invention provides a method oftreating wastewater. The method can comprise steps of promotingconditions in the wastewater to form or maintain a complexing species ofsilica and fluoride, removing at least one undesirable species from thewastewater while promoting conditions to form or maintain a complexingspecies of silica and fluoride, adjusting the conditions to inhibit theformation of the complexing species of silica and fluoride afterremoving at least one undesirable species from the wastewater, andremoving any residual undesirable species from the wastewater to producea treated effluent after adjusting the conditions to inhibit theformation of the complexing species. In accordance with one or moreembodiments of the present invention, FIG. 1 shows a wastewatertreatment system 10, which can comprise a first pretreatment system 12fluidly, connected to a wastewater, influent, in wastewater source 14.Wastewater treatment system 10 can further comprise a secondpretreatment system 16 fluidly connected to first pretreatment system12. A first separation system 18 and a second separation system 20 istypically fluidly connected downstream of first and/or secondpretreatment systems 12 and 16. Treated wastewater, effluent, typicallyundergoes further treatment in final treatment system 22 prior totransfer to discharge 24.

Influent can be any source of wastewater suitable for treatment inaccordance with the present invention. For example, a suitable influentwastewater can be wastewater accumulated having a relatively acidic pHsuch as those from phosphate manufacturing operations.

The first pretreatment system can comprise one or more unit operationsthat remove organic matter, such as algae as well as reduce theturbidity of the influent wastewater stream at its pH. A suitablepretreatment system can comprise a clarifier having ballastedflocculation subsystems. FIG. 2 shows one such exemplary unit having acoagulation stage, a maturation stage, a settling stage and ahydrocyclone. The clarifier 30 can utilize a disinfectant, such assodium hypochlorite, to deactivate any microorganisms or organic matterin the wastewater stream; a coagulating agent, such as, but not limitedto, bentonite, aluminum sulfate, and ferric chloride, to promotecoagulation of deactivated matter; and a flocculating agent such as, butnot limited to, nonionic, cationic, anionic polymers or combinationsthereof, to promote flocculation of the deactivated, coagulated matter.The clarifier can utilize microsand enhanced settling and hydrocyclonetechniques to separate sludge or solids from the liquid-rich stream.Such systems preferably reduce the turbidity of the wastewater stream toless than about 3 NTU.

The second pretreatment system comprises one or more unit operationsthat remove fine solids and/or improve the turbidity of the wastewaterstream. A suitable system can comprise a multimedia filter utilizing anyof anthracite, sand, and garnet. Such systems preferably reduce theturbidity of wastewater to less than about 2 NTU and reduce the SDI toless than about 4 to reduce the likelihood of downstream fouling.

The first and second separation systems remove contaminants orundesirable species from the wastewater to render it suitable fordischarge into a body of water. As used herein the phrase suitable fordischarge refers to treated wastewater having contaminant concentrationsthat meet or exceed United States EPA discharge requirements. Forexample, the first and second separation systems can comprise one ormore reverse osmosis devices suitable for service in conditions of thewastewater. Effluent treated wastewater typically has contaminantconcentrations as listed in Table 1.

TABLE 1 Effluent Quality Requirements (in mg/l) ConstituentConcentration pH 6.5-8.5 Fluoride <5.0 Ammonia <1.0 Total Nitrogen <2.0Phosphorus <0.5 TDS <50

Thus, in accordance with one or more embodiments of the presentinvention, first separation system 18 can comprise one or more reverseosmosis apparatus having separation membranes (not shown) suitable forservice treatment of wastewater, such as brackish water, having a pH ofless than about 3, and flux rates of about 6 to about 12 GFD because, itis believed, high flux rate greater than about 12 GFD can lead tofouling and flux rates less than about 6 GFD can lead to low permeatequality. Similarly, second separation system 20 can comprise one or morereverse osmosis apparatus 20 having separation membranes (not shown)suitable for service treatment of wastewater, such as brackish water,having a pH of about 6 to about 7 and flux rates of about 15 to about 20GFD. As with the reverse osmosis system of the first separation system,higher flux rates can lead to unacceptable fouling whereas lower fluxrates can lead to poor permeate quality. Any reverse osmosis apparatusmay be utilized in the first or second separation system. Suitableexamples include those commercially available from United States FilterCorporation, Milton, Ontario, Canada. Membranes suitable for service inthe reverse osmosis apparatus in accordance with the present inventioninclude FILMTEC BW30-365 membrane available from FilmTec, a subsidiaryof The Dow™ Chemical Corporation, Midland, Mich. The first separationsystem can be operated to treat wastewater having a pH of less thanabout 3.5 to promote the formation and/or removal of bisulfate speciesto inhibit the formation of sulfate species and reduce the scalingpotential of calcium sulfate. The first separation system can also beoperated to treat wastewater having a pH of less than about 3.5 topromote the formation and/or removal of hydrofluorosilicic species toreduce the scaling potential of silica and calcium fluoride or both. Thefirst separation system can also be operated to treat wastewater havinga pH of less than about 3.5 to promote the formation and/or removal ofphosphoric acid species to reduce the scaling potential of calciumphosphate. The first separation system can also be operated to treatwastewater having a pH of less than about 3.5 to reduce the scalingpotential of metals. The first separation system can also be operated totreat wastewater having a pH of less than about 3.5 to promote theformation and/or removal of ammonium species to improve the ammoniarejection rate. The second separation system can be operated to treatwastewater having a pH of about 6 to about 7 to promote the formationand/or removal of fluoride species to improve the removal of suchspecies. The second separation system can be operated to treatwastewater having a pH of about 6 to about 7 to promote the formationand/or removal of silicate species to improve the removal of suchspecies. The second separation system can be operated to treatwastewater having a pH of about 6 to about 7 to promote the formationand/or removal of phosphate species to improve the removal of suchspecies. The second separation system can be operated to treatwastewater having a pH of about 6 to about 7 to promote the formationand/or removal of organic species to improve the removal of suchspecies. Other techniques may be utilized in the first and secondseparation system to remove contaminants or otherwise undesirablespecies including, but not limited to, electrodialysis,electrodeionization, microfiltration, and evaporation/condensation. Insome cases, the wastewater treatment system can further comprise anantiscalant and/or a flocculating agent source disposed to introduce anantiscalant and/or a flocculating agent into the wastewater upstream ofthe pretreatment system or either of the separation systems. Anysuitable antiscalant can be used that inhibits the formation of scale inthe various unit operations in accordance with the present invention.The antiscalant can be used as recommended by respective manufacturersbut are typically introduce at a concentration of about 3 to about 4ppm. Final treatment system 22 can comprise one or more unit operationsthat further reduce any contaminant or undesirable species from thetreated wastewater and make it suitable for discharge. For example,final treatment system 22 can comprise one or more mixed-bed polishersthat reduce ammonia concentration to less than about 1 mg/1. The 15mixed-bed typically can comprise one or more anionic and cationic ionexchange resins that attract and bind residual charged species in thetreated wastewater. The ion exchange resin can be present in themixed-bed in any suitable arrangement to further purify the treatedwastewater. Examples of suitable ion exchange resins include the DOWEX™MARATHON™ resin family, available from The Dow™ Chemical Corporation,Midland, Mich., as well as the AMBERLITE™ resin family available fromRohm and Haas Company, Philadelphia, Pa. Wastewater treatment system 10typically further includes an acid source 26 and an alkali source 28.Acid source 26 is typically connected to an inlet stream of firstseparation system 18 and alkali source 28 is typically connected to aninlet stream of second separation system 20. In such an arrangement,acid from acid source 26 can adjust one or more chemical properties ofwastewater to be treated in first separation system 18. For example, thepH of wastewater to be treated in an inlet 30 of first separation system18 can be adjusted to control and/or maintain the solubility orequilibrium of one or more chemical species including, for example,inhibiting formation of precipitating species by, for example,increasing the solubility of such species and/or promoting the formationof a complexing species comprising such otherwise precipitating species.

In accordance with one or more embodiments of the present invention, anacid can be introduced into inlet 30 and mixed with wastewater to betreated to promote, maintain, or otherwise alter equilibrium conditionsto inhibit the formation of any sulfate (SO₄ ⁻²) species and/or favorthe formation of any bisulfate (HSO₄) species. As shown in FIG. 3, theequilibrium relative composition of sulfate and bisulfate species variesas a function of pH. Lower pH conditions can promote the formation ofbisulfate species whereas higher pH conditions can promote the formationof sulfate species. Thus, controlling the pH can affect the availabilityof sulfate species that typically have a tendency to precipitate in theseparation systems of the present invention.

In other embodiments, acid addition can be utilized to promote, maintainor otherwise alter equilibrium conditions to promote the formation ofhydrofluorosilicic acid and/or inhibit precipitation of silica andfluoride species. As shown in FIG. 4, the equilibrium relativecomposition of hydrofluoric acid and fluoride species varies as afunction of pH. Lower pH conditions can promote the formation ofhydrofluoric acid species whereas higher pH conditions can promote theformation of fluoride species. Thus, controlling the pH can affect theavailability of hydrofluoric acid species, which, in turn, can affectthe formation of hydrofluorosilicic species and reduce the availabilityof precipitating silica or silicate species.

In still other embodiments, acid addition can be utilized to promote,maintain, or otherwise alter equilibrium conditions to promote thesolubility phosphate species such as, but not limited to, calciumphosphate. For example, the pH of wastewater to be introduced in inlet30 of first separation system 18 can be maintained or adjusted to belowabout 3, typically to below about 2.8, and in some cases to below about2.5, and in yet other cases to about 2.

Any acid can be used in accordance with the present invention thatserves to lower or maintain the pH of a stream to the desired pH range.Suitable examples include hydrochloric acid and sulfuric acid ormixtures thereof. The selection of the particular acid will depend onseveral factors, including but not limited to, availability and cost aswell as other disposal considerations. For example, hydrochloric acidmay be preferable over sulfuric acid to avoid any concentrationincreases of the sulfate species.

Likewise, an alkali from alkali source 28 can be utilized to adjust oneor more chemical properties of wastewater to be treated in secondseparation system 20. As with acid addition, alkali addition can beadvantageously utilized to control and/or maintain the solubility orequilibrium of one or more chemical species. For example, the pH ofwastewater treated from first separation system 18 can be adjusted topromote the formation of silicate or fluoride species, or both, tofacilitate removal thereof from the wastewater stream in secondseparation system 20. Similarly, the pH can be adjusted to favor theformation of phosphate and ammonia species to facilitate removal thereoffrom the wastewater stream in second separation system 20. Thus, inaccordance with one or more embodiments of the present invention, the pHof wastewater in an inlet 32 of second separation system 20 can beraised to at least about 6, in some cases to at least about 6.5, and instill other cases to between about 6 and about 7. The pH increase canalso facilitate the formation of organic salt and their removal thereofin second separation system 20 to improve the TOC quality of theeffluent. As shown in FIG. 5, the equilibrium relative composition ofammonium and ammonia species varies as a function of pH. Lower pHconditions can promote the formation of ammonia species, which canpromote removal thereof in the first separation system. In addition, asshown in FIG. 6, the equilibrium relative composition of phosphoric acidand phosphate species varies as a function of pH. The pH conditions canbe controlled to promote the formation of H₂PO₄ ⁻ species, which canpromote removal thereof in the second separation system. Any alkali canbe used in accordance with the present invention that serves to raisethe pH of a stream to the desired pH range. Examples suitable for use asalkali include caustic soda or sodium hydroxide, caustic potash orpotassium hydroxide. Preferably, the acid and the alkali comprisespecies that are suitable for discharge to a body of water. As usedherein the terms contaminants and undesirable species refer to speciesin the wastewater or treated wastewater that have a definedconcentration limit. Contaminants include, for example, calcium,magnesium, sodium, potassium, aluminum, barium, ammonium, bicarbonate,sulfate, chloride, phosphate, nitrate, fluoride, silica, iron, andmanganese comprising species. As used herein, the term organic mattercan include bacteria, microorganisms, algae as well as suspended solidscomprising such matter. Also as used herein, the term deactivatingrefers to rendering organic matter suitable for coagulation and/orflocculation. The function and advantage of these and other embodimentsof the present invention will be more fully understood from the examplebelow. The following example is intended to illustrate the benefits ofthe present invention, but do not exemplify the full scope of theinvention.

EXAMPLE

This example shows the operation of a wastewater treatment system inaccordance with one or more embodiments of the present invention. Inparticular, the wastewater treatment system 10, schematically shown inFIG. 1, had pretreatment systems 14 and 16 comprised of a clarifier anda multimedia filter, respectively. The wastewater treatment systemfurther included a first separation system 18 comprised of a firstreverse osmosis apparatus and a second separation system 20 comprised ofa second reverse osmosis apparatus. The treatment system also includedfinal treatment system 22 comprised of a mixed-bed polisher.

The clarifier comprised of an ACTIFLO® treatment system, available fromOTV SA, and utilized NaOCl to deactivate, at least partially, anyorganic matter. The clarifier also utilized bentonite to promotecoagulation of the deactivated organic matter at about 80 to about 250mg/1, depending on the amount necessary to coagulate the organic matter.A nonionic polymeric agent, P1142 high molecular weight polymer fromBetz Dearborn, Downers Grove, Ill., was also utilized in the clarifierto promote flocculation of the coagulated, deactivated organic matter.The flocculating agent was introduced at a concentration of about 1mg/1. Effluent from the clarifier had a turbidity of less than about 3NTU. Sludge and other semisolid waste from the clarifier was returned tothe accumulation pond or otherwise disposed.

The multimedia filter utilized media comprised of anthracite, sand andgarnet to reduce the turbidity of the wastewater to less than about 2NTU and to reduce the SDI to less than about 4.

The mixed-bed polisher utilized a mixed-bed of DOWEX™ MARATHON™ A andDOWEX™ MARATHON™ C ion exchange resins, each available from The Dow™Chemical Corporation, Midland, Mich. The mixed-bed polisher served tofurther control the concentration of NH₃ to below about 1 mg/1, toreduce the concentration of PO₄ species to below about 0.5 mg/1.

The first reverse osmosis apparatus utilized FILMTEC™ BW30-365 membranesfrom FilmTec Corporation, a subsidiary of The Dow™ Chemical Corporation,Midland, Mich. It was operated at an average flux rate of about 10 GFDat about 250-300 psig operating pressure. The second reverse osmosisapparatus also utilized FILMTEC™ BW30-365 membranes. It was operated atan average flux rate of about 18 GFD. If necessary, acid (hydrochloricacid) was added from an acid source to the influent wastewater streambefore treatment in the first reverse osmosis apparatus to control thepH to below about 3. Alkali, sodium hydroxide, was added to thewastewater stream after the first reverse osmosis apparatus and beforeintroduction into the second reverse osmosis apparatus to raise the pHto between about 6 and about 7. Influent wastewater was retrieved froman accumulation pond of a phosphate manufacturing facility. It typicallyhad contaminant concentrations as listed in Table 2. The pH of thewastewater influent into the first reverse osmosis apparatus wasadjusted or maintained at between about 2 to 2.8 to maintain or promotethe complexing of silica and fluoride to form hydrofluorosilicic acidspecies thereby reducing the scaling potential associated with silicaand calcium fluoride. The pH conditions also served to shift equilibriumto favor the formation of phosphoric acid, calcium bisulfate andammonium species and consequently reduced the scaling potentialassociated with calcium phosphate and calcium sulfate while promotingremoval of ammonia. Table 2 lists the properties, including thecontaminant concentrations, of the permeate stream from the firstreverse osmosis apparatus (First Pass Permeate Composition). Table 2also lists the properties and contaminant concentrations of the permeatestream from the second reverse osmosis apparatus (Second Pass PermeateComposition). The data show that the systems and techniques of thepresent invention can be used to treat wastewater and produce aneffluent suitable for discharge that meets or exceeds EPA waterdischarge requirements. This example also illustrated the use of awastewater treatment system that had lower costs relative to traditionalsystems while avoiding lime sludge and other pretreatment chemicaldisposal.

TABLE 2 Wastewater Composition (in mg/1 unless indicated). First PassSecond Pass Influent Permeate Permeate Constituent CompositionComposition Composition Calcium 551 0.25 0.1 Magnesium 229 0.074 0.025Sodium 1,290 50.7 1.4 Potassium 196 0.86 0.021 Aluminum 8.4 0.05 0.05Barium 0.02 0.001 0.001 Ammonium 600 5.2 0.27 Bicarbonates 0.78 — 2.4Sulfates 5,200 5.5 0.2 Chlorides 100 14 0.26 Phosphates 1,600 1.1 0.004Nitrates 0.26 0.16 0.014 Fluorides 150 35 0.54 Silica 200 0.61 0.3 Iron5.6 0.02 0.025 Manganese 2.9 0.006 0.005 TDS 11,500 111 15 TSS 24 4 —BOD 17 0.74 0.2 TOC 66 1.0 0.55 TKN 650 5.9 1 pH 2.8 2.9 6.3 Turbidity(NTU) 14 0.25 0.05 Color (PCU) 110 5 5

While several embodiments of the invention have been described andillustrated herein, those of ordinary skill in the art will readilyenvision a variety of other systems and structures for performing thefunctions and/or obtaining the results or advantages described herein,and each of such variations or modifications is deemed to be within thescope of the present invention. More generally, those skilled in the artwould readily appreciate that all parameters, dimensions, materials, andconfigurations described herein are exemplary and that actualparameters, dimensions, materials, and configurations depend uponspecific applications for which the teachings of the present inventionare used. Thus, the size and capacity of each of the unit operationswould vary depending on several considerations specific to aninstallation. Further, the particular materials of construction of thevessels, pumps, and other components of the system of the presentinvention would be dependent also on particular, specific installationconsiderations but the selection, construction, and design of suchcomponents and systems would be within the scope of those skilled in theart. For example, those skilled in the art would recognize thatstainless steel should be used as materials of construction of unitoperations for service or applications where carbon steel would beunsuitable. Those skilled in the art will recognize, or be able toascertain, using no more than routine experimentation, equivalents tothe specific embodiments of the invention described herein. It is,therefore, understood that the embodiments disclosed herein arepresented by way of example only and that, within the scope of theappended claims and equivalents thereto; the invention may be practicedotherwise than as specifically described. The present invention isdirected to each individual feature, system, material and/or methoddescribed herein. In addition, any combination of two or more suchfeatures, systems, materials and/or methods, if such features, systems,materials and/or methods are not mutually inconsistent, is includedwithin the scope of the present invention. As used herein, alltransitional phrases such as “comprising,” “including,” “having,”“containing,” “involving,” and the like are open-ended, i.e. to meanincluding but not limited and only the transitional phrases “consistingof and “consisting essentially of shall be closed or semi-closedtransitional phrases, respectively, as set forth in § 2111.03 of theUnited States Patent Office Manual of Patent Examining Procedures.

1. A method of removing contaminants from an influent wastewater stream,the method comprising: a. initially chemically adjusting a pH of theinfluent wastewater stream to less than about 3.5 or maintaining the pHof the influent stream at less than about 3.5; b. after initiallyadjusting or maintaining the pH of the influent wastewater stream,directing the wastewater to a first reverse osmosis system and removingcontaminants from the wastewater; c. directing the wastewater from thefirst reverse osmosis system to a second reverse osmosis system andremoving contaminants from the wastewater; and d. adjusting the pHupwardly after the wastewater has been subjected to treatment in thefirst reverse osmosis system and prior to treatment in the secondreverse osmosis system.
 2. The method of claim 1 including adjusting thepH of the wastewater upwardly to at least about 6 or higher before thewastewater is directed through the second reverse osmosis system.
 3. Themethod of claim 2 including maintaining the wastewater effluent from thefirst reverse osmosis system at about 6 or higher prior to thewastewater being directed through the second reverse osmosis system. 4.The method of claim 1 wherein the wastewater to be treated includesfluorides, silica, phosphates, calcium, and sulfates, and wherein afterinitially adjusting the pH of the influent wastewater stream to lessthan about 3.0, maintaining the pH of the influent wastewater to lessthan about 3.0 prior to entry into the first reverse osmosis system soas to condition the influent wastewater stream to favor the formation ofcomplexing species of silica and fluoride and which further favor theformation of bisulfates, thereby reducing the scaling potential in thefirst reverse osmosis system due to silica, calcium fluoride, calciumsulfate, calcium phosphate or metals.
 5. The method of claim 4 whereinadjusting the pH of the wastewater effluent from the first reverseosmosis system upwardly forms conditions in the wastewater that favorthe formation of fluoride and silicate ions or converts any weaklyionized acids into salt form; and wherein at least 90% of the fluorides,silica, phosphates, calcium and sulfates are removed by the first andsecond osmosis systems.
 6. The method of claim 5 wherein the wastewaterbeing treated also includes ammonia, phosphates or metals and whereininitially adjusting the pH of the influent wastewater stream comprisescontrolling the pH of the influent wastewater stream to condition thewastewater to favor the formation of phosphoric acid and ammonium ionsthat reduce scaling potential in the first reverse osmosis system due tocalcium phosphate and improves ammonia removal in the first reverseosmosis system; and wherein adjusting the pH upwardly of the wastewatereffluent from the first reverse osmosis system conditions the wastewaterto favor the formation of phosphate ions which contribute to the removalof phosphates in the second reverse osmosis system and furtherconditions the wastewater to generally increase the solubility oforganics and thereby contributes to the removal of the organics in thesecond reverse osmosis system.
 7. A method of influent wastewater andremoving fluorides and silica from the influent wastewater, the methodcomprising: a. inhibiting conditions in the influent wastewater thatpromote the formation of at least one of fluoride ions or silicate ionsand promoting conditions in the wastewater that favor the formation ofhydrofluorosilicic acid by initially chemically adjusting a pH ofinfluent wastewater to less than about 3.5 and thereafter maintainingthe pH of the influent wastewater to less than about 3.5; b. removingfluorides and silica in the form of hydrofluorosilicic acid from thewastewater by directing the wastewater through a first reverse osmosissystem; c. after initially adjusting the pH of the influent wastewaterto less than about 3.5 and after the wastewater has been treated in thefirst separation system, promoting conditions that favor the formationof fluoride and silicate ions by adjusting the pH of the wastewaterupwardly to at least about 6 or higher; and d. directing the wastewater,including the fluoride and silicate ions, through a second reverseosmosis system and removing from the wastewater the fluoride andsilicate ions.
 8. A method of removing fluorides from influentwastewater comprising: a. directing the wastewater to a first reverseosmosis system and removing fluorides from the wastewater; b. prior tothe wastewater entering the first reverse osmosis system, chemicallyadjusting or maintaining the pH of the wastewater to less than about3.5; c. directing the wastewater from the first reverse osmosis systemto a second reverse osmosis system and removing fluorides from thewastewater; d. adjusting the pH of the wastewater upwardly after thewastewater has been subjected to treatment in the first reverse osmosissystem and prior to treatment in the second reverse osmosis system; ande. removing at least 90% of the fluorides from the wastewater with thefirst and second reverse osmosis systems.
 9. The method of claim 8wherein the influent wastewater also includes calcium, sulfates,phosphates and silica, and wherein at least 90% of the calcium,sulfates, phosphates and silica are removed by the first and secondreverse osmosis systems.
 10. The method of claim 8 including filteringthe wastewater prior to the wastewater being directed to the firstreverse osmosis system.
 11. The method of claim 8 wherein the influentwastewater includes an initial pH and wherein the method includesmaintaining the pH of the wastewater at or below its initial pH at leastuntil the wastewater has been treated by the first reverse osmosissystem.
 12. The method of claim 8 including adjusting the pH of thewastewater upwardly to at least about 6 or higher before the wastewateris directed through the second reverse osmosis system.
 13. The method ofclaim 8 wherein the wastewater includes silica, phosphates, calcium andsulfates and wherein the method includes conditioning the wastewaterprior to entry into the first reverse osmosis system to favor theformation of complexing species of silica and fluoride and which furtherfavor the formation of bisulfates, thereby reducing the scalingpotential in the first reverse osmosis system due to silica, calciumfluoride, calcium carbonate, calcium sulfate, calcium phosphates andmetal scales.
 14. The method of claim 13 wherein adjusting the pH of thewastewater effluent from the first reverse osmosis system upwardlyconditions the wastewater to favor the formation of fluoride andsilicate ions or converts any weakly ionized acids into salt form. 15.The method of claim 11 wherein the wastewater includes calcium, ammoniaand phosphates or metals, and wherein the method includes conditioningthe wastewater prior to entry into the first reverse osmosis system tofavor the formation of phosphoric acid and ammonium ions and that reducescaling potential in the first reverse osmosis system due to calciumphosphate and improves ammonia removal in the first reverse osmosissystem; and conditioning the wastewater effluent from the first reverseosmosis system to favor the formation of phosphate ions which contributeto the removal of phosphates in the second reverse osmosis system andfurther conditions the wastewater to generally increase the solubilityof organics and thereby contributes to the removal of the organics inthe second reverse osmosis system.
 16. The method of claim 8 wherein thewastewater includes calcium, phosphates, and sulfates, and the methodfurther comprises conditioning the wastewater to reduce the potentialfor the formation of calcium fluoride, calcium phosphate or calciumcarbonate and calcium sulfate.
 17. The method of claim 16 whereinadjusting the pH of the wastewater effluent from the first reverseosmosis system upwardly causes fluorides and silica in the wastewater toassume the form of fluoride and silicate ions which are removed from thewastewater in the second reverse osmosis system.
 18. The method of claim16 wherein the wastewater also includes sulfates, phosphates andammonia, and wherein prior to entering the first reverse osmosis systemthe wastewater is conditioned to favor the formation of bisulfates,phosphoric acid and ammonium ions, and wherein adjusting the pH of thewastewater effluent from the first reverse osmosis system upwardlyconditions the wastewater to favor the formation of phosphate ions andgenerally increases the ionization of some organics which contribute tothe removal of phosphates, organics, and ammonia from the wastewater.19. The method of claim 8 wherein the wastewater originates from aninfluent wastewater stream having a pH of 3.5 or less, and wherein thewastewater is subject to one or more pretreatments upstream of the firstreverse osmosis system, and wherein the method includes maintaining thepH of the wastewater at 3.5 or below as the wastewater passes throughthe one or more pretreatments and prior to the wastewater entering thefirst reverse osmosis system.
 20. The method of claim 19 wherein the pHof the wastewater is maintained at 3.5 or below until the wastewaterexits the first reverse osmosis system.